Method and apparatus for dual notch ripple filtering

ABSTRACT

An apparatus includes a self-coupled transformer, a first band stop filter and a tank circuit connected in series with the first band stop filter between first and second windings of the self-coupled transformer. An inductor of the tank circuit acts as an out-of-phase third winding of the self-coupled transformer.

BACKGROUND

1. Technical Field

Embodiments of the invention relate generally to power electronics.Particular embodiments relate to apparatus and methods for filtering thevoltage output from gradient power amplifiers that are used in magneticresonance imaging (MRI) systems.

2. Discussion of Art

Generally, the quality of images produced by an MRI system will beaffected by the repeatability and fidelity of its electronic components.In particular, gradient subsystem power amplifiers strongly influencethe fidelity with which a scan volume is voxellated (scanned in volumesegments of equal size and common orientation). Power amplifier rippleor oscillation can degrade a desired uniformity of voxel size andorientation.

Accordingly, MRI systems are provided with apparatus for correctingimages in response to deviations in the performance of electroniccomponents such as the gradient subsystem power amplifiers. One suchapparatus is a ripple cancellation filter, which is provided to reduceswitching noise produced at the gradient coil caused by pulse widthmodulating the gradient power supply. Typically, ripple cancellationfilters are designed to cancel noise around a single fundamentalfrequency that is driven by a pulse width modulation (PWM) switchingfrequency of the MRI system in which the filters are installed. In casean MRI system might be operated at any of plural PWM switchingfrequencies, then plural ripple cancellation filters are installed.

In view of the above, it is desirable to provide apparatus and methodsfor efficiently implementing a multi-notch ripple cancellation filterthat is usable in MRI systems capable of operating at plural PWMswitching frequencies. Such apparatus and methods might also be helpfultoward filtering the outputs of PWM power supplies, generally.

BRIEF DESCRIPTION

Embodiments of the invention provide an apparatus that includes aself-coupled transformer; a band stop filter; and a tank circuitconnected in series with the band stop filter between first and secondwindings of the self-coupled transformer. An inductor of the tankcircuit acts as an out-of-phase third winding of the self-coupledtransformer. Accordingly, the apparatus may provide a dual notch ripplecancellation filter across the self-coupled transformer.

Other embodiments provide an apparatus that includes a self-coupledtransformer having first, second, and third windings that may beoperatively connected in series between higher and lower potential inputterminals, with the second winding connected out-of-phase to the firstand third windings; a band stop filter connected in series between thesecond and third windings of the self-coupled transformer; a firsttuning capacitor connected in parallel across the second winding of theself-coupled transformer; and output terminals operatively connectedbetween the first and second windings and between the second and thirdwindings.

Other embodiments implement a method that includes connecting first,second, and third windings of a self-coupled transformer in seriesacross terminals of a pulse width modulated power supply, with one ofthe windings out-of-phase to the other windings; connecting a firsttuning capacitor across the out-of-phase winding of the self-coupledtransformer; connecting a band stop filter in series between theout-of-phase winding of the self-coupled transformer and one of theother windings; and connecting a load across the band stop filter andthe first tuning capacitor, so that the band stop filter, the firsttuning capacitor, and the out-of-phase winding provide a dual notchripple cancellation filter between the pulse width modulated powersupply and the load.

DRAWINGS

The present invention will be better understood from reading thefollowing description of non-limiting embodiments, with reference to theattached drawings, wherein below:

FIG. 1 shows schematically an exemplary magnetic resonance imaging (MRI)system in which an embodiment of the present invention is implemented.

FIG. 2 shows schematically a gradient power amplifier and a ripplecancellation filter used in the exemplary MRI system of FIG. 1.

FIG. 3 shows graphically a dual notch frequency rejection image to beprovided by a dual notch ripple cancellation filter according to anembodiment of the invention.

FIG. 4 shows schematically a dual notch ripple cancellation filteraccording to an embodiment of the invention.

DETAILED DESCRIPTION

Reference will be made below in detail to exemplary embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference characters usedthroughout the drawings refer to the same or like parts, withoutduplicative description. Although exemplary embodiments of the presentinvention are described with respect to gradient amplifiers and gradientcoils used in MRI systems, embodiments of the invention also areapplicable for use with pulse width modulated (PWM) power supplies,generally.

As used herein, the terms “substantially,” “generally,” and “about”indicate conditions within reasonably achievable manufacturing andassembly tolerances, relative to ideal desired conditions suitable forachieving the functional purpose of a component or assembly.

FIG. 1 shows major components of an exemplary magnetic resonance imaging(MRI) system 10 configured for use with embodiments of the presentinvention. The operation of the system is controlled from an operatorconsole 12, which includes a keyboard or other input device 13, acontrol panel 14, and a display screen 16. The input device 13 caninclude a mouse, joystick, keyboard, track ball, touch activated screen,light wand, voice control, or any similar or equivalent input device,and may be used for interactive geometry prescription. The console 12communicates through a link 18 with a separate computer system 20 thatenables an operator to control the production and display of images onthe display screen 16. The computer system 20 includes a number ofmodules that communicate with each other through a backplane 20 a. Themodules of the computer system 20 include an image processor module 22,a CPU module 24 and a memory module 26 that may include a frame bufferfor storing image data arrays. The computer system 20 is linked toarchival media devices, permanent or back-up memory storage or a networkfor storage of image data and programs, and communicates with a separateMRI system control 32 through a high-speed signal link 34. The computersystem 20 and the MRI system control 32 collectively form an “MRIcontroller” 33. According to embodiments and aspects of the invention,the MRI controller 33 is configured to accomplish a method forseparately imaging water, fat, and silicone, for example by implementingan exemplary algorithm that is further discussed below.

The MRI system control 32 includes a set of modules connected togetherby a backplane 32 a. These include a CPU module 36 as well as a pulsegenerator module 38. The CPU module 36 connects to the operator console12 through a serial link 40. It is through link 40 that the MRI systemcontrol 32 receives commands from the operator to indicate the scansequence that is to be performed. The CPU module 36 operates the systemcomponents to carry out the desired scan sequence and produces datawhich indicates the timing, strength and shape of the RF pulsesproduced, and the timing and length of the data acquisition window. TheCPU module 36 connects to several components that are operated by theMRI controller 33, including the pulse generator module 38 (whichcontrols a gradient amplifier 42, further discussed below), aphysiological acquisition controller (“PAC”) 44, and a scan roominterface circuit 46.

The CPU module 36 receives patient data from the physiologicalacquisition controller 44, which receives signals from a number ofdifferent sensors connected to the patient, such as ECG signals fromelectrodes attached to the patient. And finally, the CPU module 36receives from the scan room interface circuit 46, signals from varioussensors associated with the condition of the patient and the magnetsystem. It is also through the scan room interface circuit 46 that theMRI controller 33 commands a patient positioning system 48 to move thepatient or client C to a desired position for the scan.

The pulse generator module 38 operates the gradient amplifiers 42 toachieve desired timing and shape of the gradient pulses that areproduced during the scan. The gradient waveforms produced by the pulsegenerator module 38 are applied to the gradient amplifier system 42having Gx, Gy, and Gz amplifiers. Each gradient amplifier excites acorresponding physical gradient coil x, y, or z in a gradient coilassembly, generally designated 50, to produce the magnetic fieldgradients used for spatially encoding acquired signals. The gradientcoil assembly 50 forms part of a magnet assembly 52, which also includesa polarizing magnet 54 (which, in operation, provides a homogeneouslongitudinal magnetic field B0) and a whole-body RF coil 56 (which, inoperation, provides a transverse magnetic field B1 that is generallyperpendicular to B0). In an embodiment of the invention, RF coil 56 is amulti-channel coil. A transceiver module 58 in the MRI system control 32produces pulses that are amplified by an RF amplifier 60 and coupled tothe RF coil 56 by a transmit/receive switch 62. The resulting signalsemitted by the excited nuclei in the patient may be sensed by the sameRF coil 56 and coupled through the transmit/receive switch 62 to apreamplifier 64. The amplified MR signals are demodulated, filtered, anddigitized in the receiver section of the transceiver 58. Thetransmit/receive switch 62 is controlled by a signal from the pulsegenerator module 32 to electrically connect the RF amplifier 60 to thecoil 56 during the transmit mode and to connect the preamplifier 64 tothe coil 56 during the receive mode. The transmit/receive switch 62 canalso enable a separate RF coil (for example, a surface coil) to be usedin either transmit mode or receive mode.

After the multi-channel RF coil 56 picks up the RF signals produced fromexcitation of the target, the transceiver module 58 digitizes thesesignals. The MRI controller 33 then processes the digitized signals byFourier transform to produce k-space data, which then is transferred toa memory module 66, or other computer readable media, via the MRI systemcontrol 32. “Computer readable media” may include, for example,structures configured so that electrical, optical, or magnetic statesmay be fixed in a manner perceptible and reproducible by a conventionalcomputer: e.g., text or images printed to paper or displayed on ascreen, optical discs, or other optical storage media; “flash” memory,EEPROM, SDRAM, or other electrical storage media; floppy or othermagnetic discs, magnetic tape, or other magnetic storage media.

A scan is complete when an array of raw k-space data has been acquiredin the computer readable media 66. This raw k-space data is rearrangedinto separate k-space data arrays for each image to be reconstructed,and each of these is input to an array processor 68 which operates toFourier transform the data into an array of image data. This image datais conveyed through the serial link 34 to the computer system 20 whereit is stored in memory. In response to commands received from theoperator console 12, this image data may be archived in long-termstorage or it may be further processed by the image processor 22 andconveyed to the operator console 12 and presented on the display 16.

As mentioned above, during operation of the MRI system 10 for an MRIscan, the pulse generator module 38 applies gradient waveforms to thegradient coil assembly 50 via the gradient amplifier system 42. Thegradient waveforms drive corresponding gradient coils to locally adjustmagnetization of a scan volume enclosed by the magnet assembly 52. Inparticular, the gradient waveforms provide Frequency Encoding, PhaseEncoding, and Slice Selection gradients of magnetization in order todefine a specific region of interest for an MRI experiment within themagnet assembly 52.

The gradient amplifier system 42 includes three gradient amplifiers, oneper gradient axis (X, Y, Z). FIG. 2 shows schematically a gradientamplifier 200 that is formed as a stacked topology of plural H-bridgecircuits 202.1, 202.2, . . . 202.n. The stacked H-bridges 202 arecomposed of IGBTs 204, which are driven by a PWM controller 205according to a pulse width modulation algorithm that trades offswitching and conductive losses of the IGBTs and bridge interleaveschemes. Generally, pulse width modulation (PWM) is a process of turningselected IGBTs on and off, according to a programmed schedule, in orderto produce a time-averaged voltage from a DC power supply to a load. Thefraction of a PWM schedule for which a device is on is defined as thatdevice's duty cycle. The frequency at which the devices are turning onand off is defined as the PWM switching frequency Fsw of the controller.Although in some schedules (e.g., when PWM is used to simulate AC) thedurations of on and off pulse times may vary across a schedule, theswitching frequency at which the IGBTs toggle remains constant, i.e.,the IGBTs can change state only at an integral multiple of Fsw. TheH-bridges 202 are stacked to achieve the required maximum output voltageand in certain embodiments their PWM schedules are interleaved tominimize output filtering requirements.

As mentioned, imaging performance of the MRI system 10 can be influencedby the repeatability and fidelity of the gradient subsystem poweramplifiers 200. Therefore, in addition to interleaving PWM schedules, inan embodiment, a ripple cancellation filter 206 is connected across theoutput terminals of the stacked H-bridges 202 in order to mitigate anyinfluence of the gradient amplifier 200 on imaging performance. Thegradient amplifier 200 drives its gradient coil x, y, or z (50 x.y.z)via the ripple cancellation filter 206. The ripple cancellation filter206 is configured to monitor output frequency of the pulse widthmodulation (PWM) controller 205, and to cancel switching noise producedfrom the stacked H-bridges 202 at harmonics of the PWM frequency.

FIG. 3 shows graphically a dual notch frequency rejection image 300 tobe provided by the ripple cancellation filter 206, according to anembodiment of the invention. In the depicted embodiment, the frequencyrejection image 300 includes first and second notches 302, 304 thatindicate particularly strong attenuation of signal passing through thefilter. The output of the ripple cancellation filter 206 (currentsupplied to the gradient coil 50) is generally equal to the input to theripple cancellation filter 206 (bridge current) multiplied by thefrequency rejection image 300. By superimposing the notches 302, 304onto fundamental frequencies of PWM switching noise, the ripplecancellation filter 206 can provide a clean power supply to the gradientcoil 50 according to the frequency encoding, phase encoding, and sliceselection gradient waveforms as selected by an operation of the MRIcontrol system 32.

FIG. 4 shows schematically components of the dual notch ripplecancellation filter 206 according to an embodiment of the invention. Theripple cancellation filter 206 includes higher and lower potential inputterminals 402, 403 and higher and lower potential output terminals 404,405 by which the higher and lower potential filter legs 406, 408 can beoperatively connected between the gradient power amplifier 200 and thegradient coil 50. The ripple cancellation filter 206 also includes aself-coupled choke transformer 410 and ladder filter 412, which areconnected across the filter legs 406, 408 in parallel to a load ladder414.

In the depicted embodiment, the choke transformer 410 includes a firstwinding 416, which has its high end connected to the higher potentialinput terminal 402 and has its low end connected to feed the higherpotential filter leg 406. The transformer 410 further includes a secondor filter winding 418, which has its low end connected to the higherpotential filter leg 406 and has its high end connected to tap theladder filter 412. The transformer 410 also includes a third winding420, which has its low end connected to the lower potential inputterminal 403 and has its high end connected to the lower potentialfilter leg 408.

The ladder filter 412 is connected between the higher and lowerpotential filter legs 406, 408 in parallel to the load ladder 414. Theladder filter 412 includes a first tuning capacitor 422, a tuninginductor 424, and a second tuning capacitor 426. Values of the ladderfilter components are selected so that the ladder filter 412 will affectthe dual notches 302, 304 of the frequency rejection image 300 as shownin FIG. 3. For example, values of the first tuning capacitor 422 and ofthe tuning inductor 424 can be selected to define a band stop filterfrequency, and values of the filter winding 418 and of the second tuningcapacitor 426 can be selected relative to the first and second windings416, 420, the first tuning capacitor 422, and the tuning inductor 424 todefine a band-pass filter frequency that matches and partly cancels theband stop filter frequency, thereby defining the dual notches 302, 304as side bands of the band-pass filter.

Advantageously, the dual notch ripple cancellation filter 206 can beconfigured to provide additional notches in the frequency rejectionimage 300, by adding band-pass and band stop steps onto the ladderfilter 412.

One aspect of the invention is that the coupling of the filter winding418 with the first and third windings 416, 420 alters the frequencyresponse of the tank circuit that would otherwise be formed by thefilter winding 418 in parallel with the second tuning capacitor 426.

In embodiments, the load ladder 414 includes resistors 428 andcapacitors 430, which are arranged in parallel and have their valuesselected to allocate voltages among the higher and lower potentialoutput terminals 404, 405 and the center output terminal 431 (which canbe ground-connected to balance the gradient coil 50).

Embodiments of the invention provide an apparatus that includes aself-coupled transformer; a first band stop filter; and a tank circuitconnected in series with the band stop filter between first and secondwindings of the self-coupled transformer. An inductor of the tankcircuit acts as an out-of-phase third winding of the self-coupledtransformer. Accordingly, the apparatus may provide a dual notch ripplecancellation filter across the self-coupled transformer. The apparatusalso may include a load ladder of resistors and capacitors connected inparallel across the out-of-phase third winding and the band-stop filter,and the load ladder may be tapped for high, low, and center outputs. Thecapacitances of the load ladder may be less than the capacitances of theband-stop filter and the tank circuit. The self-coupled transformerwindings may have inductances smaller than the inductance of theband-stop filter. The out-of-phase winding of the self-coupledtransformer may have an inductance larger than the other windings. Theout-of-phase winding of the self-coupled transformer may have aninductance smaller than the other windings. The out-of-phase winding ofthe self-coupled transformer may have an inductance larger than theinductance of the band-stop filter. The out-of-phase winding of theself-coupled transformer may have an inductance smaller than theinductance of the band-stop filter. The self-coupled transformerwindings may have inductances larger than the inductance of theband-stop filter. The apparatus may also include at least one additionalband stop filter connected in parallel with the first band stop filter.

Other embodiments provide an apparatus that includes a self-coupledtransformer having first, second, and third windings that may beoperatively connected in series between higher and lower potential inputterminals, with the second winding connected out-of-phase to the firstand third windings; a first band stop filter connected in series betweenthe second and third windings of the self-coupled transformer; a firsttuning capacitor connected in parallel across the second winding of theself-coupled transformer; and output terminals operatively connectedbetween the first and second windings and between the second and thirdwindings. The apparatus also may include a load ladder of resistors andcapacitors connected across the output terminals; wherein the loadladder may be tapped for high, low, and center outputs. The capacitancesof the load ladder may be less than the capacitances of the band-stopfilter and the first tuning capacitor. The self-coupled transformerwindings may have inductances smaller than the inductance of theband-stop filter. The out-of-phase winding of the self-coupledtransformer may have an inductance larger than the other windings. Theout-of-phase winding of the self-coupled transformer may have aninductance smaller than the other windings. The out-of-phase winding ofthe self-coupled transformer may have an inductance larger than theinductance of the band-stop filter. The out-of-phase winding of theself-coupled transformer may have an inductance smaller than theinductance of the band-stop filter. The self-coupled transformerwindings may have inductances larger than the inductance of theband-stop filter. The apparatus may also include at least one additionalband stop filter connected in parallel to the first band stop filter.

Other embodiments implement a method that includes connecting first,second, and third windings of a self-coupled transformer in seriesacross terminals of a pulse width modulated power supply, with one ofthe windings out-of-phase to the other windings; connecting a firsttuning capacitor across the out-of-phase winding of the self-coupledtransformer; connecting a first band stop filter in series between theout-of-phase winding of the self-coupled transformer and one of theother windings; and connecting a load across the band stop filter andthe first tuning capacitor, so that the band stop filter, the firsttuning capacitor, and the out-of-phase winding provide a dual notchripple cancellation filter between the pulse width modulated powersupply and the load. The method also may include connecting at least oneadditional band stop filter in parallel to the first band stop filter,wherein the at least one additional band stop filter provides at leastone additional notch for ripple cancellation.

It is to be understood that the above description is intended to beillustrative, and not restrictive. For example, the above-describedembodiments (and/or aspects thereof) may be used in combination witheach other. In addition, many modifications may be made to adapt aparticular situation or material to the teachings of the inventionwithout departing from its scope. While the dimensions and types ofmaterials described herein are intended to define the parameters of theinvention, they are by no means limiting and are exemplary embodiments.Many other embodiments will be apparent to those of skill in the artupon reviewing the above description. The scope of the invention should,therefore, be determined with reference to the appended claims, alongwith the full scope of equivalents to which such claims are entitled. Inthe appended claims, the terms “including” and “in which” are used asthe plain-English equivalents of the respective terms “comprising” and“wherein.” Moreover, in the following claims, terms such as “first,”“second,” “third,” “upper,” “lower,” “bottom,” “top,” etc. are usedmerely as labels, and are not intended to impose numerical or positionalrequirements on their objects. Further, the limitations of the followingclaims are not written in means-plus-function format and are notintended to be interpreted based on 35 U.S.C. §112, sixth paragraph,unless and until such claim limitations expressly use the phrase “meansfor” followed by a statement of function void of further structure.

This written description uses examples to disclose several embodimentsof the invention, including the best mode, and also to enable one ofordinary skill in the art to practice embodiments of the invention,including making and using any devices or systems and performing anyincorporated methods. The patentable scope of the invention is definedby the claims, and may include other examples that occur to one ofordinary skill in the art. Such other examples are intended to be withinthe scope of the claims if they have structural elements that do notdiffer from the literal language of the claims, or if they includeequivalent structural elements with insubstantial differences from theliteral language of the claims.

As used herein, an element or step recited in the singular and proceededwith the word “a” or “an” should be understood as not excluding pluralof the elements or steps, unless such exclusion is explicitly stated.Furthermore, references to “one embodiment” of the present invention arenot intended to be interpreted as excluding the existence of additionalembodiments that also incorporate the recited features. Moreover, unlessexplicitly stated to the contrary, embodiments “comprising,”“including,” or “having” an element or a plurality of elements having aparticular property may include additional such elements not having thatproperty.

Since certain changes may be made in the above-described apparatus andmethods, without departing from the spirit and scope of the inventionherein involved, it is intended that all of the subject matter of theabove description or shown in the accompanying drawings shall beinterpreted merely as examples illustrating the inventive concept hereinand shall not be construed as limiting the invention.

What is claimed is:
 1. An apparatus comprising: a self-coupledtransformer; a first band stop filter; and a tank circuit connected inseries with the band stop filter between first and second windings ofthe self-coupled transformer; wherein an inductor of the tank circuitacts as an out-of-phase third winding of the self-coupled transformer.2. The apparatus of claim 1 wherein the apparatus provides a dual notchripple cancellation filter across the self-coupled transformer.
 3. Theapparatus of claim 2, further comprising: a load ladder of resistors andcapacitors connected in parallel across the out-of-phase third windingand the band stop filter; wherein the load ladder is tapped for high,low, and center outputs.
 4. The apparatus of claim 3 wherein thecapacitances of the load ladder are less than the capacitances of theband-stop filter and the tank circuit.
 5. The apparatus of claim 2wherein the self-coupled transformer windings have inductances smallerthan the inductance of the band stop filter.
 6. The apparatus of claim 2wherein the out-of-phase winding of the self-coupled transformer has aninductance larger than the other windings.
 7. The apparatus of claim 2wherein the out-of-phase winding of the self-coupled transformer has aninductance smaller than the other windings.
 8. The apparatus of claim 2wherein the out-of-phase winding of the self-coupled transformer has aninductance larger than the inductance of the band stop filter.
 9. Theapparatus of claim 2 wherein the out-of-phase winding of theself-coupled transformer has an inductance smaller than the inductanceof the band stop filter.
 10. The apparatus of claim 2 wherein theself-coupled transformer windings have inductances larger than theinductance of the band stop filter.
 11. The apparatus of claim 1 furthercomprising at least one additional band stop filter connected inparallel with the first band stop filter.
 12. An apparatus comprising: aself-coupled transformer having first, second, and third windings thatare operatively connected in series between higher and lower potentialinput terminals, with the second winding connected out-of-phase to thefirst and third windings; a first band stop filter connected in seriesbetween the second and third windings of the self-coupled transformer; afirst tuning capacitor connected in parallel across the second windingof the self-coupled transformer; and output terminals operativelyconnected between the first and second windings and between the secondand third windings.
 13. The apparatus of claim 12 further comprising atleast one additional band stop filter connected in parallel to the firstband stop filter.
 14. The apparatus of claim 12, further comprising: aload ladder of resistors and capacitors connected across the outputterminals; wherein the load ladder is tapped for high, low, and centeroutputs.
 15. The apparatus of claim 12 wherein the self-coupledtransformer windings have inductances smaller than the inductance of theband stop filter.
 16. The apparatus of claim 12 wherein the out-of-phasewinding of the self-coupled transformer has an inductance larger thanthe other windings.
 17. The apparatus of claim 12 wherein theout-of-phase winding of the self-coupled transformer has an inductancelarger than the inductance of the band stop filter.
 18. The apparatus ofclaim 12 wherein the self-coupled transformer windings have inductanceslarger than the inductance of the band stop filter.
 19. A methodcomprising: connecting first, second, and third windings of aself-coupled transformer in series across terminals of a pulse widthmodulated power supply, with one of the windings out-of-phase to theother windings; connecting a first tuning capacitor across theout-of-phase winding of the self-coupled transformer; connecting a firstband stop filter in series between the out-of-phase winding of theself-coupled transformer and one of the other windings; and connecting aload across the first band stop filter and the first tuning capacitor;wherein the first band stop filter, the first tuning capacitor, and theout-of-phase winding provide a dual notch ripple cancellation filterbetween the pulse width modulated power supply and the load.
 20. Themethod of claim 19 further comprising connecting at least one additionalband stop filter in parallel to the first band stop filter, wherein theat least one additional band stop filter provides at least oneadditional notch for ripple cancellation.